add more retry logic to watch service

[cjh1]

We want to prevent intermittent error such as a 502 from the RESTful endpoint causing the watch look to terminate here.

[cjh1]

For example we should try to recover from this:

    raise self._exception
  File "/home/kitware/.pyenv/versions/3.9.7/envs/distiller/lib/python3.9/site-packages/tenacity/_asyncio.py", line 51, in __call__
    result = await fn(*args, **kwargs)
  File "/home/kitware/distiller/cli/watch/distiller/modes/scan_4d_files.py", line 79, in post_file_event
    async with session.post(
  File "/home/kitware/.pyenv/versions/3.9.7/envs/distiller/lib/python3.9/site-packages/aiohttp/client.py", line 1117, in __aenter__
    self._resp = await self._coro
  File "/home/kitware/.pyenv/versions/3.9.7/envs/distiller/lib/python3.9/site-packages/aiohttp/client.py", line 381, in _request
    raise RuntimeError("Session is closed")
RuntimeError: Session is closed